Richard Kuklinkski, a killer-for-hire who carried out contracts for several mafia families.
I think it is safe to assume that no one really knows how many people this guy actually killed. I suspect that there are still plenty of bones strewn across the United States that no one knows about.
If you have the stomach for it, the one or two books that have been written about Richard's life are really interesting, and there is plenty of source material that could easily be assimilated into one's setting. Gruesome stuff, for sure.